(1) Field of the Invention
This invention concerns solderable contact materials, and relates in particular to methods of providing these on substrates therefor.
(2) Description of the Prior Art
There are numerous occasions in the general electronics field where it is required to solder an electrical connection to a component or a lead thereto via a contact pad (an area of conductive material itself already connected to the component or lead). For example, semiconductor devices--including light--emitting diodes and hybrid assemblies--may be mounted on a non-conductive substrate and have thin conductive leads extending therefrom to relatively large pads which may in turn be connected, by wires soldered thereto, to the pins of a dual-in-line package of which the device and its substrate form a part. Again, the electrodes of a liquid crystal display unit may be connected via thin conductive leads to larger pads by which the unit may be solder-wire connected to the equipment driving the display. Yet again, the conductive tracks of a printed circuit board, to which tracks the components carried by the board are all connected, may be considered as a series of giant contact pads.
It is at present conventional to vacuum deposit contact pads into place in a preliminary stage of the device's manufacture. Unfortunately, making contacts in this way has presented problems, for vacuum-deposited contacts are commonly of materials that tend to oxidize during any subsequent heat processing of the device. Thus, one contact material combination used is a bi-layerof NICHROME (NICHROME is a Registered Trademark for various alloys of nickel and chromium; NICHROME IV is 80% Ni, 20% Cr, while NICHROME wire or ribbon is commonly either 75% Ni, 12% Fe, 11% Cr and 2% Mn, or 60% Ni, 24% Fe, 16% Cr), as a base adhesion layer, and then aluminum. Even at room temperature the aluminum forms a thin oxide layer, and can only be ultrasonically bonded to aluminum or gold wire; at elevated temperatures (i.e., 200.degree. C.), the aluminum forms a thicker oxide layer which renders the pad useless. Another combination is a bi-layer of chromium or NICHROME and then gold, but this requires a reasonably thick layer of gold for it to be bondable or solderable, so a tri-layer combination of chromium or NICHROME followed by copper and then gold has been used and has been quite successful. However, the cost of gold has led to the investigation of other alloys and combinations with varied results.
In the Specification of our copending British Application for Letters Patent No: 83/12,213 (Publication No: 2,139,248) (I/6766/MB) it is disclosed that certain alloys of copper containing very small ( 0.5 wt%) quantities of aluminum may be used on top of a base adhesion layer of chromium or NICHROME to construct (by, for example, standard evaporation techniques) bi-layer contact pads--the layer of chromium or NICHROME, and then the layer of aluminum/copper alloy--that are both solderable and remain so even when ovened in air at 200.degree. (and so do not oxidize under such conditions). The present invention concerns the very similar use of some different copper alloys, namely the use of alloys of copper and manganese.